The invention is an improvement in the fuel-air charging system for a loop charged two-stroke engine typical of those used in outboard motors. Such systems usually include at least two main input transfer passages and one or more boost passages. Over the years much effort has gone into improving scavenging in these engines by either enlarging, multiplying or changing the shape of such passages as well as the pistons and combustion chambers to achieve a better flow of fresh charge into the engine and thereby gain more power and increased efficiency. An example of this effort in regard to cross-charged engines is U.S. Pat. No. 3,494,335 issued to H. R. E. Meier in 1968.
As in all well developed arts, there is a continuing effort not only to improve performance but also to simplify design and develop methods of manufacture to reduce cost. This invention is a step forward in each of these areas as it teaches a highly efficient cylinder and piston combination designed specifically to be compatible with high pressure die casting of aluminum engine cylinder blocks.
One problem encountered in designing a cylinder of the type described herein, where the input passages are channels in the cylinder wall, is how to direct at least a portion of the incoming charge across the face of the piston -- as well as up into the combustion chamber -- and thereby more effectively scavenge the burned charge. In the prior art this problem has been solved typically by forming the input passage within the cylinder block which provided lateral room for the passage to be curved from upright to horizontal before discharging into the cylinder. This cannot be done with passages open to the cylinder, consequently, prior to the invention, performancce was sacrificed to gain the cost advantage of this latter construction. The piston and cylinder design of the invention overcomes this deficiency while preserving the economy of construction.